Cross-channel probe system for time domain reflectometry detection of fluid flow

ABSTRACT

A sensor system for use with time domain reflectometry systems to allow measurement of relative proportions of intermixed constituents having differing electrical permittivities contained in a fluid mixture. The sensor comprises at least one primary electrode and one secondary electrode which are spaced apart across a flow channel. One electrode is connected to a first conductor carrying the active time domain reflectometry signal pulse. The other electrode is connected to the passive or ground conductor which carries any reactive signal resulting from the active signal. A fluid mixture of variable proportions will demonstrate a variable dielectric constant which affects the reflected signals sensed by the time domain reflectometer.

TECHNICAL FIELD

The technical field of this invention is sensing apparatus used withtime domain reflectometry systems to determine the relative proportionsof mixed fluids, particularly mixed liquid and gaseous phases, forexample mixtures of water and steam.

BACKGROUND OF THE INVENTION

Time domain reflectometry has been previously known effective in methodsfor determining the level of a liquid, such as in a tank. According tosuch time domain reflectometry methods, electrical pulses are conveyedalong a transmission line to an electrically conductive probe extendingover the range of liquid levels being detected. The stimulatingelectrical pulses produced in the time domain reflectometry system arepartially reflected at the vapor-liquid interface due to a change in theelectrical impedance. The impedance change is associated with thedifferences in the dielectric strength between the liquid and theoverlying gas or vapor. The electrical permittivity is the technicalterm indicating the dielectric properties of the fluids involved.

The electrical pulses produced by a time domain reflectometry system areaffected by the dielectric constant of the surrounding media in whichthe signal is traveling. The dielectric constant (permittivity) of theadjacent media directly affects the propagation velocity of anelectromagnetic wave as it travels along the transmission line and alongany attached probe or sensor. In time domain reflectometry systems, afast rise time electromagnetic pulse is propagated along a transmissionline having a known length while measuring the time of arrival and thetime of reflections from electrical discontinuities in the transmissionline at two known, spaced points. One known, spaced point is locatedwhere a coaxial connecting cable of the transmission line is attached tothe transmission line probe. The other known, spaced point is located atthe distal end of the transmission line probe. Since these locations areboth known, one can calculate the propagation velocity of theelectromagnetic wave and, as a result, calculate the apparent dielectricconstant of the material undergoing tests and through which thetransmission line probe extends. Similarly, changes in the dielectricconstant which relate to changes in the media adjacent the probe canalso be determined. For example, the apparent dielectric constant mayprovide a direct indication of the presence of water versus the presenceof water vapor or air.

U.S. Pat. No. 4,786,857 to Charles L. Mohr, et al., entitled "Methodsand Apparatus for Time Domain Reflectometry Determination of RelativeProportion, Fluid Inventory and Turbulence", disclosed apparatus andmethods for using time domain reflectometry to determine the relativeproportions of intermixed constituents in a fluid system. Such apparatusand methods can be used to determine the relative proportions of liquidand vapor even when the liquid and vapor are intermixed eitherhomogeneously or non-homogeneously. Measurement capabilities such asthese are particularly valuable to the process industries and nuclearenergy production. The systems can be used to monitor nuclear reactorcoolant systems, in which the total inventory of system coolant,including intermixed water and steam, must be determined under a varietyof conditions, including even accident conditions. Methods are alsodescribed for obtaining indications of turbulence in fluid mixtures bymeasuring variations in fluid properties over time.

The above-mentioned Mohr patent disclosed a probe including an innercentrally located electrode mounted within a cylindrical outerelectrode. The cylindrical outer electrode was provided with slots toallow fluid to pass into the annular volume between the inner and theouter electrodes. The probe was immersed in the mixed-constituentsystem. The average dielectric constant or permittivity experienced bythe electrical pulse transiting the probe was determined using timedomain reflectometry. The measured permittivity was then correlated withknown characteristic data of the constituents being measured todetermine their relative proportions.

U.S. Pat. No. 5,554,936, also to Charles L. Mohr, et al., entitled"Mixed Fluid Time Domain Reflectometry Sensors", disclosed apparatus inthe form of improved probe sensors which could be used for a greatervariety of applications and still provide measurements. Moreparticularly, there was a need to provide a probe that was moreeffective when used in some applications, particularly in applicationswhere solutions rich in minerals, such as from earth wells, were notcapable of measurement. Accordingly, the improved probe sensor wascapable of service under a variety of conditions with accuracy andreliability.

The probe shown in U.S. Pat. No. 5,554,936 has been found less thansatisfactory when used in some situations. One situation is when theprobe is required to be placed directly within the flow path of a fluidflow channel. Placement of the probe sensor directly across a flowchannel subjects the probe to pressures from fluid flow, increases therisk of damage from the flowing fluid and materials present within suchflow. Placement across a flow channel also requires that the probesensor be removed during cleaning operations of the fluid flow channelin order to prevent damage to such probe. The current inventionaddresses the need for improved time domain reflectometry probes whichare capable of service under a greater variety of conditions in fluidflow channels, with accuracy and reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more preferred forms of the invention are described herein withreference to the accompanying drawings. The drawings are brieflydescribed below.

FIG. 1 is a perspective view of a time domain reflectometry sensorsystem for measuring flow characteristics in accordance with a firstpreferred embodiment of the invention.

FIG. 2 is an end view of one cross-channel sensor forming part of thesensor system of FIG. 1.

FIG. 3 is a simplified schematic and partial sectional plan viewillustrating sensors and other key parts of the system of FIG. 1.

FIG. 4 is an enlarged longitudinal sectional view taken along sectionline 4--4 of FIG. 2.

FIG. 5 is an exploded perspective view of one sensor forming part of thesystem of FIGS. 1-4.

FIG. 6 is a partial sectional view taken along line 6--6 of FIG. 2.

FIG. 7 is a graph showing system measurements for impedance for variousdelay times.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws "to promote the progressof science and useful arts" (Article 1, Section 8).

FIG. 1 illustrates a time domain reflectometry cross-channel probe orsensor system 10 in accordance with a preferred embodiment of theinvention. The probe or sensor system 10 is intended to be used withtime domain reflectometry systems to allow measurement of relativeproportions of intermixed constituents having differing electricalpermittivities contained in a fluid mixture. An example of such a timedomain reflectometry system is described in U.S. Pat. No. 4,786,857 toCharles L. Mohr, entitled "Methods and Apparatus for Time DomainReflectometry Determination of Relative Proportion, Fluid Inventory andTurbulence", which is hereby incorporated by reference in its entirety.Another example of sensors for use in such a time domain reflectometrysystem is described in U.S. Pat. No. 5,554,936, also to Charles L. Mohr,entitled "Mixed Fluid Time Domain Reflectometry Sensors", which is alsohereby incorporated by reference in its entirety.

Probe or sensor system 10 advantageously includes a pair of probes orsensors 12 and 14 supported in opposing walls 16 and 18. Walls 16 and 18form parts of a fluid flow channel 20 in which a fluid mixture 22 iscontained. More specifically, walls 16 and 18 form fluid guiding orcontaining walls that partially define fluid flow channel 20.

Probe 12 supports a primary electrode 24 in fluid channel-forming wall16. In the preferred construction shown, the inside face of electrode 24has a sensing surface 26. The inside face of electrode 24 is positionedadjacent to fluid mixture 22. The inside face can either be immediatelyadjacent to the fluid mixture or separated by a thin dielectric layer.It is preferable to use a thin dielectric layer to reduce signal loss,particularly when the flow channel contains fluids which areelectrically conductive, or contain ions which can electrically affectthe signal applied to the electrode. The inside face is most preferablyin approximately level relationship with the associated channel-formingwall 16. Primary electrode 24 is supported in electrically isolatedrelationship within wall 16.

Secondary electrode 28 is supported in an electrically isolatedcondition within wall 18 in a fashion similar to electrode 24. Secondaryelectrode 28 is also spaced and electrically isolated from primaryelectrode 24. Secondary electrode 28 is supported in wall 18 such that asensing surface 30 is supported adjacent to fluid mixture 22 insubstantially level relation with the associated channel-forming wall18.

The primary and secondary electrodes are preferably positioned inface-to-face opposing relationship. In the preferred configurationshown, primary electrode 24 and secondary electrode 28 are each formedin the shape of elongated pieces having longitudinal axes 53 (FIG. 2)which for both electrodes are oriented in the same plane. Each ofelectrodes 24 and 28 also advantageously has a local recess and aperturefor facilitating electrical connection with accompanying electricalconductors (see FIG. 6 at conductor 64). Other connection constructionsare also possible.

The fluid mixture 22 or other composition being measured is interposedbetween primary electrode 24 and secondary electrode 28. In manyinstances, this will involve a moving fluid which is interposed betweenthese electrodes. One instance of use is in the measuring of relativeproportions of intermixed constituents within a flowing fluid mixture22. It is important to note that primary electrode 24 and secondaryelectrode 28 are mounted within walls 16 and 18, respectively, in amanner which does not inhibit or impede fluid flow of fluid mixture 22within fluid flow channel 20. Hence, fluid flow characteristics are notimpeded by the presence of probe system 10 along channel 20. This alsofacilitates maintenance and cleaning of channel 20 without requiring theremoval of probe system 10 therefrom.

Fluid channel walls 16 and 18 are presented in a substantially parallelconfiguration such that probes 12 and 14 are mounted in a substantiallyco-linear relation. In such a relationship, the primary electrode 24 andsecondary electrode 28 are in complementary and opposing positions alongopposite sides of fluid flow channel 20. It is to be understood thatadditional fluid channel-forming walls are provided for joining togetherwalls 16 and 18, but are not shown here, so as to encircle channel 20and contain fluid mixture 22 inside channel 20. One commonimplementation includes a single cylindrical pipe or duct, with probes12 and 14 being provided in complementary and opposing wall portions.This allows the probe system to detect the electrical permittivity ofconstituents of fluid mixture 22, such as when such constituents areflowing by and between the opposing probe faces. In this manner or anyof a number of other cross-channel configurations, the probe system 10of this invention can be utilized.

According to the probe construction of probe system 10 and as shown inFIG. 1, fluid mixture 22 is interposed between probes 12 and 14 so as toprovide, at least in part, a dielectric layer therebetween. Preferably,a separate dielectric face layer is also provided on probes 12 and 14.The face layer can be formed as a coating upon the inward faces ofprimary electrode 24 and secondary electrode 28. The dielectric facelayer covers the sensing surfaces 26 and 30, respectively. Details ofsuch a coating system are described below with reference to FIG. 2.

As shown in FIGS. 1 and 3, channel probe system 10 includes a pair ofsubstantially matched lead lines 32 and 34. Probes 12 and 14 areconnected in parallel with a signal main line 36 via a tee connector 38.Signal main line 36 supplies a time domain reflectometry pulse via teeconnector 38 to lead lines 32 and 34 and to probes 12 and 14. Thecentral or first conductor of the signal main line is connected to thecentral conductors of both lead lines 32 and 34. The outer or secondconductor of the main signal line 36 is connected to the outer or secondconductors of lead lines 32 and 34. In the preferred arrangement of thisinvention, the primary electrode 24 is connected to the center conductorof lead 32, whereas the secondary electrode 28 is connected to the outeror second conductor of lead line 34. This provides an opposing polarityrelationship between the primary electrode 24 and secondary electrode28.

Lead lines 32 and 34, main signal line 36, tee connector 38, andelectrical portions of probes 12 and 14, including electrodes 24 and 28,are preferably constructed and sized so as to substantially impedancematch the components. Impedance matching minimizes the occurrence of anyspurious or unwanted impedance-induced signal reflections resulting froma signal being transmitted therethrough. Additionally, by substantiallymatching the lengths of lead lines 32 and 34, the resultant timing ofsignal delivery to and from electrodes 24 and 28 will be matched.

In operation, secondary electrode 28 is connected to the second,passive, or ground conductor of signal line 36. Primary electrode 24 isconnected with the primary or active signal conductor of line 36. Lines32, 34 and 36 can also include a third and outer shield conductor whichis provided merely to shield both the active and passive signalconductors from undesirable electrical field interference and thusminimize errors from stray electromagnetic sources.

Secondary electrode 28 of probe 14 produces a ground reaction whichreacts or interacts with the active pulse from primary electrode 24.When the active pulse and reactive pulse reach the electrodes 24 and 28,this develops a field across the fluid mixture within channel 20. Thesignal pulses propagate at a very high speed, so the matching of cablelength on lines 32 and 34 is essential to monitoring the reflectedsignals which result from the field and impedance experienced across thechannel between electrodes 24 and 28.

It is also desirable to impedance match essentially all of thecomponents of system 10. To implement this, it is also preferable tomatch impedances at tee 38 and lines 32 and 34 versus the impedancealong line 36. Signal line 36 is preferably provided with a 50-ohmnominal line impedance. Signal line 36 connects with a signal processorto deliver a signal pulse to system 10. The stimulating signal pulse isdriven down the 50-ohm main signal line 36 and is then split at teeconnector 38. Preferably, lead lines 32 and 34 ideally each have a100-ohm line impedance in order to provide in parallel an effectiveimpedance which matches or nearly matches the 50-ohm signal line.However, the bulk of commercially available low-cost coaxial linestypically have line impedances in the range of 80-90 ohms, and such adegree of matching has been found acceptable. For those applicationswhere cost is a consideration, such lines will prove suitable whenattempting to substantially impedance match the signal lines connectedto electrodes 24 and 28. By driving a pulse down the 50-ohm signal line36 and splitting it at tee connector 38, an effective impedance ofapproximately 40 ohms results. The resulting active and reactive pulsesboth transit along branch lines 32 and 34.

Probe 14 is coupled with the ground lead by a metal outer sheath of line34. The active lead of line 32 is advantageously provided by the centerconductor of line 32, and is coupled with primary electrode 24 ofprimary probe 12. The secondary conductor of line 32 is not connected toany of the sensory components of probe 12, but instead dead-ends at apoint removed from the sensor 24 electrode. Conversely, the centerconductor on lead line 34 is not connected to any of the sensorycomponents of probe 14. It also dead-ends at a point removed from thesensing electrode 28. Further details of such arrangement are shown withrespect to FIG. 3.

As shown on FIGS. 1 and 3, bifurcated lead lines 32 and 34, signal line36, and tee connector 38 comprise coaxial electrical signal cablingcomponents. In this manner, each of probes 12 and 14 is preferablyconnected to a coaxial electrical signal lead such as coaxial cables 32and 34, respectively. Coaxial cables 32 and 34 each have a central, orinner, conductor and a secondary conductor which can be in the form ofan outer conductor 48 (see FIG. 4). Alternatively, and more preferably,the secondary conductor can be sheathed by a third shielding layeroutside of the secondary conductor.

Primary conductor 46 of lead line, or cable, 32 is electricallyconnected to primary electrode 24 of probe 12. Outer sheathing 48 oflead line, or cable, 34 is electrically connected to secondary electrode28 of probe 14. Because probes 12 and 14 are contemplated for use in ahigh temperature environment, the cables of lines 32, 34 and 36 have astainless steel outer sheathing, silica insulation, and copper primaryand secondary conductors. This design for lead lines 32, 34 and 36 issometimes referred to as "hardline" coaxial cable.

A time domain reflectometry active signal has a very sharp rise and dropin voltage. This is transmitted along the central conductor to primaryelectrode 24 of probe 12. The active signal experiences a detectiblechange in impedance where the secondary conductor 48 ends within theprobe housing 50. More specifically, this occurs at the shoulder 73 ofpart 74 as shown in FIG. 4. The permittivity at this point changes andthe time domain reflectometry detection system will show a noticeablechange. This point thus serves as a reference which serves to helpdelineate between the line portion of the time domain reflectometerrange and a transition portion. The transition portion extends from suchpoint outwardly to the connection with the electrode 24. Other signalreference points can also be used if there is a sufficient detectableimpedance change. For example, the proximate end of the electrode isconnected to conductor 64 at a point which may give rise to a detectibleimpedance change. In prior testing, this point has not been as easilyidentified in the time domain reflectometry scans as is the terminus ofthe secondary conductor near shoulder 73. These or other points alongthe electrical path can be used as timing reference points.

The invention considers either the detected change in impedance betweenthe electrodes 24 and 28, or transit time associated with transit of asignal between a reference point and the distal ends 25 of theelectrodes (see FIG. 4). Where the end of the secondary conductor isused as a reference point, then there are two segments of time, onebetween shoulder 73 and the end 64 of the conductor where it meets theelectrode. The other is the segment along electrode 24 itself betweenthe proximal end 67 and the distal end 25 of the electrodes. Thepermittivity of the first segment (between 73 and 67), and thepermittivity of the second segment (between 67 and 25) is themeasurement being taken by the system.

In an alternative system, the transit time from points 67 to 25 can beused if there is sufficient impedance change at point 67 to allow thispoint to be discerned from the time domain reflectometry traces. Ineither approach, the short time period between the signal reaching theproximal end 67 of primary electrode 24 and the time it reaches thedistal end is a function affected by the average permittivity of thefluid mixture or other media held in channel 20 across which the sensoris detecting.

It has also been found that the permittivity of the fluid within channel20 also has a noticeable effect on the impedance measured for thetransition segment from shoulder 73 to proximal end 67 of theelectrodes. Thus the capacitance of this segment also varies with theconstituency of the fluid channel and calibration is needed toaccurately determine the changes in reflected signal strength overvarious delay times as a function of the fluid in channel 20.

FIG. 2 illustrates cross-channel probe 12 in end view as seensubstantially flush-mounted in wall 16. More particularly, theorientation of primary electrode 24 within an end face of probe 12 isshown to extend substantially transverse to the direction of fluid flowwithin flow channel 20. Preferably, primary electrode 24 is seatedwithin an insulator block 60 formed from a ceramic, or Teflon™ plastic,or other suitable insulators. Primary electrode 24 forms a thinconductor that is mounted in insulator block 60, along the surface, justunder the surface, or along the surface with a coating (not shown)thereover. Sensing surface 26 and any over-layer are constructed andmounted to provide a substantially flush, or level, relation with theadjacent surface of insulator block 60 when received therein. Alsopreferably, insulator block 60 is formed from a substantially elongateand rectangular block of insulatory material which is received in ahousing 50 about which additional insulating material is received. Forexample, block 60 is preferably formed from a ceramic material havingsufficient resistance to erosion or ablatement, such as a high purity(99.99%) alumina or zirconia stabilized with magnesia may also beacceptable.

Block 60 and insulating pieces 41 and 43, in combination, cooperate toform a round insulating plug assembly that extends from the cylindricalshape of housing 50. Alternatively, parts 60, 41 and 43 can be a singlepiece of suitable material, such as alumina or zirconia with electrode24 attached. Preferably, the diameter of probe housing 50, where itextends through wall 16, is in the range of 1.0 to 1.5 inches indiameter, with primary electrode 24 being embedded into the pluginsulator block 60. This assembly can also be made from Teflon™ or othersuitable dielectic and chemically resistant materials depending upon theservice in which the assembly will be used.

Block 60 is designed to have a sufficient depth and width to allow thedesired electric field to be developed by primary electrode 24. This isdone in such a manner that the field will not be unduly affected by thesurrounding metal of housing 50 and wall 16.

According to one construction, the conductor depth of primary electrode24 is preferably in the range of 0.010 inches to 0.02 inches inthickness, with a geometry of 1.0 inches by 0.5 inches up to 1.0 inchesby 0.75 inches in plan view, forming sensing surface 26 accordingly.Also according to this implementation, insulator block 60 is sized inthe range of 0.2 to 0.3 inches in thickness, or depth, extendingperpendicular to sensing surface 26. Preferably, insulator block 60, aswell as insulating pieces 41 and 43, are formed from a ceramic material.Alternatively, any combination of Teflon™ or ceramics could be utilized.The conductor forming primary electrode 24 is preferably buried underthe surface of insulating block 40 to allow it to be used in thedetection of conductive fluids. Alternatively, the conductor of primaryelectrode 24 can be exposed where process conditions allow, or be coatedwith a coating material (not shown, also discussed below).

FIG. 4 illustrates the construction of sensing probe 12, enablingsubstantially flush mounting within a wall defining a fluid flowchannel. It is to be understood that sensing probe 14 is similarlyconstructed, with the exception that the ground sheath 48 or othersecondary conductor is electrically connected with the correspondingsecondary electrode 28 (see FIGS. 1 and 3).

Probe 12 is preferably formed from a cylindrical metal housing memberwhich is preferably sized by machining it to enable it to be receivedand fitted in sealed engagement with wall 16 (see FIG. 5). Housingmember 50 is formed with an enlarged and circumferentially extendingpressure seal flange 52 that cooperates to define a seal face 54 formating with a complementary seat 104 within wall 16. Seal face 54extends between pressure seal flange 52 and a cylindrical insert portion56. Insert portion 56 is sized to be snugly received within a receivingbore 102 (see FIG. 5) such that primary electrode 24 is provided in asubstantially planar relation with the inside surface of wall 16. Whenassembling housing 50 in a wall, an abutment surface 57 along sidepressure seal flange 52 is engaged with a nut 17 in the form of acompression type screw ring configured to press the probe housing member50 into the tapered seat 104 (see FIG. 5), forming a seal. A cylindricalnut guide 58 is formed by housing 50 about which such a screw ring nut17 is received.

Primary electrode 24 is presented for mounting in substantially flushrelation with a support wall of a fluid flow channel 22 by mountingelectrode 24 within a ceramic insert 60 (see FIG. 5). Ceramic insert 60is sized to be received within insert portion 56 of housing member 50such that the conductive metal electrode 24 is electrically isolatedfrom housing 50. As shown in FIG. 4, primary electrode 24 is furtherelectrically coupled with center conductor 46 of lead line 32 by way ofan intermediate conductor 62.

Conductor 62 preferably has an outer insulating cover 66 and a coppercenter, or central, conductor 64, as shown in FIG. 4. One end ofconductor 64 extends through an aperture in ceramic insert 60 and acorresponding aperture in primary electrode 24 such that one end portionis folded over and brazed into electrically conductive relation with arecess in primary electrode 24. An opposite end portion of conductor 64is brought into electrically conductive engagement with center conductor46 by brazing or otherwise connecting them within the annular connector76. Primary electrode 24 couples with center conductor 64 at proximalend 67 such that, when brazed together, they remain substantially flushwithin a receiving recess of ceramic insert 60. Hence, primary electrode24 is presented in substantially flush relation with a wall in whichprobe 12 is received.

As shown in FIG. 4, center conductor 64 of conductive line 62 issupported for electrical connection with center conductor 46 by way of asplit ceramic collar 70 and a transition insulator 74. Ceramic collar 70and transition insulator 74 are configured to engage in inter-fittingrelation, with center conductor 64 extending through a conductoraperture 72 of transition insulator 74. In this manner, center conductor64 is presented and received within annular connector 76 such thatcenter conductor 64 is presented in substantially collinear and adjacentrelation with center conductor 46 where they are joined together.

Transition insulator 74 is further supported in coaxial relation withinhousing 50 by way of a stainless steel cylindrical ferrule 78 and astainless steel mating cap 80. The joint between these two parts arepreferably welded or brazed. A transition assembly 77 is formed byinsulator 74, tube 76, ferrule 78 and cap 80 for providing electricalconnection between electrode 24 and coaxial cable 32 and such assemblyserves as a primary pressure boundary. Cap 80 is sized to receive leadline 32, with cap 80 being preferably welded or brazed to ground sheath48 in electrically conductive relation. Stainless steel ferrule 78 isthen received for coaxial insertion within cap 80, forming a rigidstructural encasement that is electrically isolated and coaxiallyencircles center conductors 64 and 46 therein. A ceramic isolator 68 isprovided for encircling the entire assembly, including ferrule 78, cap80, and split ceramic collar 70, extending within housing 50. In thismanner, electrical connection is made between center conductor 46 oflead line 32 and primary electrode 24, while electrically isolatingground sheath 48 from center conductor 64. Hence, the preceding formstransition assembly 77 so as to be positioned between coaxial cable 32and primary electrode 24 to provide electrical connection between thecoaxial cable conductor 46 and electrode 24.

In an alternative version, the transition insulator and related partsdescribed above can be replaced with a Teflon™-filled, or similarsuitable material-filled, coaxial line. This alternative constructiondoes not require the specific construction indicated because the linewill self-seal under many less severe service applications, such aswater and many chemicals under low and moderate pressure conditions.

Additionally, a seal can advantageously be provided to prevent leakagethrough or about coaxial cable 32, by way of a coaxial gland 84 (FIG.4). Gland 84 is formed by mating together a gland body member 86 and asuitably sized cap member 88. Gland body 86 forms a cylindrical surfacehaving male threads at both ends. One end is received in threadedrelationship with part 50. The other end is in threaded relationshipwith cap member 88 which has complementary female threads such that cap88 and gland body 86 can be joined together. The joint between part 50and gland body 86 can also be further sealed by welding, or it can beunthreaded and totally mounted by welding.

Cap 88 receives an insulating ferrule 90 that is sized to snugly receivelead line 32 therethrough. Ferrule 90 includes a seal part flange 92which is held within a receiving chamber of gland body 86. These partspreferably have mating conical surfaces which are complementary andforced together by a follower sleeve 95 as cap 88 is screwed onto cap 86and against follower 95.

In the above manner, lead line 32 can be sealed to housing 50 to preventleakage in applications where there is pressurized fluid in flow channel20. The transition assembly components or suitable substitutes enableelectrical connection of center conductor 46 with primary electrode 24through this sealed joint.

The construction shown in FIG. 4 is also suitable for electricallyconnecting cable 34 with secondary electrode 28 (of FIG. 1). In the caseof the other probe 14, the construction of FIG. 4 is slightly modifiedsuch that the secondary conductor in the form of ground sheath 48 iselectrically coupled with the center conductor 64 of intermediateconductor 62 by way of a stainless steel cylindrical ferrule (not shown)similar to that of ferrule 78, but which is sized to extend completelyaround split ceramic collar 70. Conductor 64 is brazed or mechanicallyengaged with ferrule 78 so as to provide electrically conductiveengagement between secondary conductor 48 and the secondary electrode28.

Electrodes 24 and 28 of FIGS. 1 and 3 are positioned relative to flowchannel 20 so as to present sensing surfaces 26 and 30 for sensing thecharacteristics of fluid mixture 22. Mixture 22 serves as a dielectricdisposed between face surfaces 26 and 30. However, it is sometimespreferred to deposit a dielectric coating on surfaces 26 and 30 toisolate electrodes 24 and 28 from the fluid mixture. Surface 26 mayinclude an end portion of center conductor 64 which connects withelectrode 24 using any suitable brazing/welding or other material usedto join electrode 24 with conductor 64. The electrode 28 is connected tothe sheath or second conductor 48. This is also accomplished in asuitable manner such as by brazing or welding.

The entire face surfaces are preferably coated with the electrodes 24and 28 covered. More preferably, a dielectric layer (not shown) isprovided to cover surfaces 26 and 30 in order to insulate and isolatethe corresponding electrodes from the fluid mixture which helps toprevent dissipation or attenuation of the stimulating time domainreflectometry signal and the resulting reflected signals, particularlywhen fluid 22 is electrically conductive.

Teflon™ polymer (polytetrafluoroethylene) is one preferred material forforming the dielectric layer. Teflon™ is preferably applied to primaryelectrode 24 and secondary electrode 28, along sensing surfaces 26 and30, respectively, by using a baked Teflon™ coating process, such as at atemperature of 750° F. The process results in a Teflon™ layer having athickness of between 0.002 and 0.005 inches, preferably about 0.003inches. The Teflon™ is applied after electrodes 24 and 28 are receivedwithin an insulator block 40, and before assembly within housing 50.

According to the construction of FIGS. 1-4, primary electrode 24 andsecondary electrode 28 are preferably fabricated from a metal, metalalloy or other suitable electrically conductive material which isadvantageously resistant to corrosion and erosion. A variety ofmaterials are suitable. Hastel™ and Zircaloy™ are examples of suitablematerials, with Zircaloy™ being preferred in high temperature, highlycorrosive environments. Zircaloy™ is a trademark for a family ofmaterials. A variety of Zircaloy™ alloys from this family can be useddepending on the process conditions. "Zircaloy™4" is currently the mostpreferred for mineralized water and steam applications.

Oxidation is another method of providing a dielectric layer overportions or all of the electrodes 24 and 28. The preferred methods ofoxidizing a Zircaloy™ electrode include subjecting it to steam in anautoclave at 400° C. at a pressure of 1,500 lbs. per square inch forapproximately 48 hours. This process creates a zirconium oxide surfacewhich is electrically non-conductive while also being highly resistantto corrosion. Such method is preferably implemented so as to coatelectrodes 24 and 28, prior to assembly within ceramic insert 60 withinhousing 50 (of FIG. 4).

Oxides can alternatively be sputtered onto the surface of the primaryand secondary electrodes. The resulting oxide coating is optionallyhardened by subsequently treating the electrode in a vacuum furnace at705° C. for two hours. The oxides will typically have thicknesses of0.001-0.010 inches.

An alternative oxidation method involves anodizing the electrodes andcenter conductor 64 (see FIG. 4) before assembly of housing 50. Theresulting anodized surface provides dielectric isolation between theelectrodes and center conductor 64 in the fluid mixture. Anodization ofa Zircaloy™ electrode is preferably accomplished in 0.05% sodiumhydroxide at 150 volts for 30 minutes. The anodized surface ispreferably removed over electrode surface areas to which components areto be welded. Alternatively, the components can be welded, then anodizedand assembled within housing 50.

The coatings or layers described above can also be combined. Forinstance, one preferable dielectric layer is obtained by providing aTeflon™ coating over an anodized surface.

Tests indicate that a baked Teflon™ coating works well in applicationswhere erosion of the electrode surface is not a problem and where fluidmixture temperatures are less than 550° F. The electrode of such a probecan be made of various nickel-based alloys to resist corrosion. Where aTeflon™ coating has very slight effect on probe impedance measurements,such an effect is constant and can be easily accounted for duringcalibration of the system. More importantly, the insulating propertiesof the Teflon™ or other dielectric layer or layers significantly reducesignal losses through attenuation where measurements are being taken inconductive fluid mixtures. In applications where erosion is a problem orwhere temperatures are greater than 550° F., an oxide coating ispreferable to Teflon™. Such an oxide coating may be created by anodizingor autoclaving as mentioned above. Oxide layers have a similar effect onimpedance measurements as does a Teflon™ coating.

FIG. 5 illustrates an exploded perspective view of the assembly ofsensor probe 12 within the receiving bore 102 to provide primary sensor24 in substantially flush relation with wall 16. Receiving bore 102 issized to receive housing member 50 such that primary sensor 24 remainssubstantially flush, or level, with wall 16. Primary sensor 24 issupported within ceramic insert 60, in combination with blocks 41 and43, for insertion within an aperture 45 of housing 50. Blocks 41 and 43are also preferably formed from an insulatory material, such as aceramic material, and engage in dovetail-fashion with ceramic insert 60,as discussed below in reference to FIG. 6. When assembled, blocks 41 and43 and ceramic insert 60 form a cylindrical insert that is snugly andsealingly received within aperture 45. In this manner, primary electrode24 is electrically insulated from metal housing 50. It is also possiblefor parts 60, 41 and 43 to be formed using a single block of Teflon™ orceramic which is brazed or otherwise formed into place.

Also according to FIG. 5, the engagement of seal face 54 on housing 50with corresponding seat 104 in wall 14 can be readily seen. Moreparticularly, the threading engagement of the compression-type screwring nut 17 into complementary threaded bore 106 of wall 16 will driveseal face 54 into engagement with seat 104, forming a seal therebetween.More particularly, a cylindrical end 96 of screw ring nut 17 is broughtinto engagement with housing 50. A hexagonal head 100 on screw ring nut17 facilitates tightening with a wrench. A through bore 97 on screw ringnut 17 receives cylindrical section 58 therethrough. In this manner,lead line 32 and a portion of housing 50 are coaxially received withinscrew ring nut 17. The use of screw ring nut 17 to removably install andseal sensor probe 12 in wall 16 (as well as nut 19 to removably installand seal sensor probe 14 in wall 16) facilitates service andmaintenance.

According to FIG. 6, intermediate conductor 62 is assembled to extendthrough ceramic insert 60, preferably with a tight, sealed fit. A pairof receiving slots 110 are also provided in either side of ceramicinsert 60 so as to provide for interdigitating assembly with blocks 41and 43 (of FIG. 5). Center conductor 64 of intermediate conductor 62 isreceived through a hole in a receiving depression of primary electrode24 where it is then folded over and welded. The presence of recess 63enables the substantially flush presentment and welding of centerconductor 64 in relation to primary electrode 24. Hence, thesubstantially flush and smooth presentment of primary electrode 24 isprovided within ceramic insert 60, and in assembly, with wall 16 (ofFIG. 5).

The invention also includes novel methods. The methods include producinga series of time domain reflectometry signals. Such signals aregenerated at suitable intervals to allow the electrical pulse or pulsescontained in the time domain reflectometry signals to transit thedistance from the signal generator (not shown) to the point or points ofreflection which generate reflected time domain reflectometry signalsand then back to the signal detector contained in the time domainreflectometer. The time domain reflectometry circuitry senses or detectsthe reflected signals and measures the strength of the reflectedsignals. This is done repeatedly at different delay times in order todetermine the impedance at various points along the conductor beingtested. With this information the reflected signal strength andeffective impedance at various distances down the line and connectedprobes are obtained. The measured voltage, or effective impedancederived therefrom, of the reflected signals allows the user to ascertainwhether an impedance mismatch exists and the delay indicates thedistance along the line at which the mismatch is occurring.

The methods further include conducting the series of time domainreflectometry signals along an electrically conductive signal line, suchas signal line 36. The conducting of the time domain reflectometrysignals is preferably done in a way which does not dissipate the signalduring conduction. Although some losses are a necessary part ofelectrical signal propagation, the inventors preferably use a coaxialconductor, such as described above to reduce or minimize signal losses.

The methods further advantageously include bifurcating the time domainreflectometry signals between a first lead line, such as line 32, and asecond lead line, such as line 34. The bifurcation of the stimulatingsignals is advantageously accomplished using an electrically conductivetee connection. Other means for bifurcation are also possible, however,at this time none are as desirable as the straightforward and reliabletee connection. Tee connection 38 is a suitable example.

The bifurcation of the stimulating signal forms time domain sub-signals.The time domain reflectometry sub-signals can each be considered asprimary and secondary branch signals. The primary branch signals includefirst portions which are the active pulses which are conducted to theprimary sensor electrode 24. The primary branch line 32 also carries anyreactive signals which are induced in the secondary conductor as aresult of the active pulses being conducted down the primary conductorof line 32. The secondary conductor maintains continuity in theimpedance experienced by the stimulating time domain reflectometrysignals. If the secondary conductor was terminated earlier in thecircuit, then there would be an impedance change associated with thepoint at which the insulation sheath and surrounding secondary conductorare stopped. This is true because the surrounding insulation andsecondary conductor have an effect upon the dielectric constantexperienced by the active signal pulses as they progress down the centerconductor of branch line 32. Such an impedance change would result in asignificant reflection of the time domain reflectometry stimulationsignal. The end of the secondary conductor of line 32 is ended withinsensor 12, and even more preferably as close as practical to theelectrode 24.

The secondary branch signals are conducted by the secondary branch line,such as line 34. The secondary conductor of branch line 34 conducts anyreactive signal induced in the secondary conductor. Such reactivesignals may be a result of direct induction from the active signalcarried on the primary conductor of line 34, or line 36 which leadsthereto. The active signal is propagated down the primary conductor ofsecondary branch line 34 to maintain continuity of the impedance of line34, as explained above with regard to the primary branch line 32. Theend of the primary conductor of line 34 is ended within sensor 14without electrical connection to electrode 28. Even more preferably, thedead end of the primary conductor of secondary branch line 34 is endedas close as practical to the electrode 28 without achieving electricalcontact.

Methods according to this invention also include detecting reflectedsignals which return from the primary and secondary electrodes. Thedetecting can be done in several different ways according to known timedomain reflectometry techniques. In essence the stimulating signals aresend down line 36, through tee 38, and down each branch line 32 and 34.The time domain reflectometer sends numerous stimulation signals andthen detects the amount of reflected signal which returns at variousdelay times to compile an estimate of the impedance along the lines.FIG. 7 show a graph having a series of traces or curves 141-145. Eachcurve indicates relative voltage of the reflected signals at variousdelay times. The relative voltage of the reflected signals alsoindicates the impedance as a function of delay time. The delay timeindicated on the X-axis of these figures also translates into anindication of distance along the conductors 36, 32 and 34, and along thelength of electrodes 24 and 28.

Curve 141 shows a plot wherein the flow channel is filled with a fluidmixture which is all or principally a gas or vapor, such as air, steam,mixtures thereof or other gases and vapors. The point labelled 151 isthe start of the transition, such as at shoulder 73. The impedanceincreases from point 151 toward a high-point 161 which represents at ornear the proximal end 67 of the electrode. The reflected signal 141 atgreater delay times then shows decreases in the impedance fromhigh-point 161 toward point 171 which represents the distal end 25 ofthe electrodes. Thereafter the associated impedance is relatively higherand is an artifact arising from reflection of the active and passivesignals from the distal ends 25 of electrodes 24 and 28.

Curves 142-145 have generally similar shapes as curve 141 justdescribed. Each curve has an associated high-point 162-165 indicatingapproximately the proximal end of the electrodes. Curves 141-145 show adecreasing dielectric constant experienced by the active and passivesignals from curve 141 to curve 145. This is associated with decreasingproportions of water or other liquid in exchange for increasing gas orvapor in the fluid mixture being tested. Thus curve 141 represents acurve for a fluid mixture which is more water and curve 145 represents acurve for a fluid mixture which is more gas or vapor.

An decrease in the dielectric constant has an associated effects ofincreasing the velocity of the electrical pulse being conveyed anddecreasing the transit period to and from a particular point along thetime domain reflectometry signal path. Decreasing dielectric constant(decreasing permittivity) also increases the apparent impedance andreflected signal voltage experienced in the transition and electrodeportions of the curves, such as at points 161 and 171 as compared topoints 165 and 175 which show higher impedance and higher reflectedvoltage signals.

The relative proportions of gas or vapor relative to the water, liquidor other mixture can be determined in two different approaches. Oneapproach utilizes the measured impedances at the electrodes, orelectrodes and transition portions of the signal. Another approachconsiders the relative transit times associated with the signals. Bothwill be discussed below.

The length along the X-axis of the time segment between points 161 and171 indicates the time needed to transit the electrodes 24 and 28between the proximal and distal ends thereof. The period of time variesas a function of the square roots of the permittivities of the fluid inchannel 20. An example is air which has a permittivity of 1 at ambienttemperatures and pressures. Water has a permittivity of approximately 80at ambient temperatures and pressures. Thus the impedance differencebetween an all air sample versus an all water sample will vary by thesquare root of 80 divided by the square root of 1, or a factor ofapproximately 9 times.

The equation relating these transit times is of the general form:

    total transit time=transit time for segment A+transit time for segment B

Transit time for segment A is determined from calibration measurementsand remains a factor in the equation as used with a particular set ofsensors and their installation. Recalibration may be neededperiodically.

Transit time for segment B is the transit time between the proximate anddistal ends of electrodes 24 and 28. This is a component of the measuredtransit time.

The transit time for segment B is estimated by deriving such from thetotal transit times being detected measured using the time domainreflectometry system. This is done by using the above relationships andcalibration testing with materials with known dielectric constants, suchas air and water. With such calibration testing the value of transittime B is known or closely estimated. Therefore the transit time forsegment A can be calculated. The transit time for segment A does nottypically vary in a significant manner between calibration testing ofthe system and normal data acquisition. This provides the basis fordetermining and estimating the transit time for segment A. Once thetransit time for segment A is sufficiently estimated, then the measuredtotal transit times can be used in the calculating of approximatetransit times for segment B.

The transit times for segment B are used to estimate the dielectricconstant detected between sensor electrodes 24 and 28 during thetesting. The derived dielectric constant measurement relates to othersimilar measurements with different materials as the ratio of the squareroots of the dielectric constants. For example the ratio in transittimes between a channel filled with air might be represented by thefollowing: ##EQU1## This type of calculation is then used to determinethe type or proportion of material or materials present between theelectrodes during testing.

Due to the relative difficulty of defining the starting point of theelectrode it may be difficult to quantitatively determine the relativeperiods of time. This occurs because the permittivity changes occur inthe transition portion of the probes as well as at the actual electrodesurfaces. Thus, to date, the relative impedance values at the electrodesegment of the reflected signals has been found to be a more reliablemeasure of the relative proportions of the constituents contained in thefluid mixture being sensed.

The relationship of relative impedance values is also a square rootrelationship. Impedance Z relates to dielectric constant C between twocases in the following relationship:

    Z.sub.1 is proportional to 1/(C.sub.1).sup.-1/2

    Z.sub.2 is proportional to 1/(C.sub.2).sup.-1/2

Thus Z₁ and Z₂ are related in the following equation:

    Z.sub.1 /Z.sub.2 =(C.sub.2).sup.-1/2 /(C.sub.1).sup.-1/2

This indicates that as the dielectric constant increases for water (C₂)versus (C₁) then the impedance increases for air (Z₁). This is shown byFIG. 7 where the higher concentrations of air are shown with curve 145and such has higher impedance values.

The novel methods of this invention further include calibrating a sensorsystem such as described above. The calibrating includes deriving anestimated transit time A which is used to correct or interpret the totaltransit time and derive an estimated transit time B.

The methods can further include deriving a measure of dielectricconstant or permittivity experienced between the probes 12 and 14. Thisderiving can be in turn used in identifying the proportions or presenceof at least one variable constituent present in the flow channel betweenthe electrodes. The system shown is also advantageous in allowing suchdeterminations to be made as a flowing fluid passes between the sensor24 and 28 and without requiring that the flow be stopped or otherwise bemodified or abridged in any significant fashion.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

What is claimed is:
 1. A sensor system for use with time domainreflectometry systems to allow measurement of a fluid mixture within afluid channel having constituents with differing electricalpermittivities, comprising:a first sensor having a first sensor housing;said first sensor being mountable upon a first side of the fluid channelwith an inside face thereof which is directed toward the fluid channel;at least one primary electrode made from electrically conductivematerial and having a sensing surface which is alone the inside face ofthe first sensor, the at least one primary electrode serving to conducta pulsed time domain reflectometry signal along the sensing surface ofthe at least one primary electrode; a second sensor having a secondsensor housing; said second sensor being mountable upon a second side ofthe fluid channel with an inside face thereof directed toward the fluidchannel in opposing relationship to the inside surface of said firstsensor; at least one secondary electrode made from electricallyconductive material and having a sensing surface which is along theinside face of the second sensor; a first lead connected to the firstsensor in electrical communication therewith, said first lead having aprimary conductor and a secondary conductor, said first lead primaryconductor being electrically connected to the at least one primaryelectrode; a second lead connected to the second sensor in electricalcommunication therewith, said second lead having a primary conductor anda secondary conductor, said second lead secondary conductor beingelectrically connected to the second electrode; a signal bifurcationconnector for connecting to said first lead and said second lead at theprimary and secondary conductors thereof, said signal bifurcationconnector further having a main lead connection for connecting to a mainlead through which time domain reflectometry signals are conveyed to andfrom the first and second sensors.
 2. A sensor system according to claim1 and further comprising at least one dielectric layer covering eitherof said sensing surfaces of the at least one primary electrode or atleast one secondary electrode, for electrically isolating at least oneof the electrodes from the fluid mixture.
 3. A sensor system accordingto claim 1 wherein the at least one primary and secondary electrodeshave elongated sensing surfaces.
 4. A sensor system according to claim 1and further comprising at least one transition assembly providingelectrical connection with at least one of said primary or secondaryelectrodes, and providing at least one seal to aid in preventing fluidleakage about said at least one of said primary or secondary electrodes.5. A sensor system according to claim 1 and further comprising at leastone transition assembly providing electrical connection with at leastone of said primary or secondary electrodes, the transition assemblyincluding a transition insulator and an intermediate conductor which isin sealed relationship received through the transition insulator to sealthe sensor from the fluid mixture.
 6. A sensor system according to claim1 wherein the first lead and second lead are approximately the samelength and have approximately the same impedance.
 7. A sensor systemaccording to claim 1 and further comprising at least one dielectriclayer covering at least the sensing surface of the at least one primaryelectrode, for electrically isolating the primary electrode from thefluid mixture and preventing dissipation of a time domain reflectometrysignal propagated to the sensor system.
 8. A sensor system according toclaim 7 wherein the at least one dielectric layer is an oxide layerformed upon the sensing surface of the at least one primary electrode.9. A sensor system according to claim 7 wherein the at least onedielectric layer covers portions of the at least one secondary electrodeto electrically isolate the at least one secondary electrode from thefluid mixture and to further prevent dissipation of a time domainreflectometry signal.
 10. A sensor system according to claim 1 andfurther comprising a main lead signal cable having a primary conductorwhich is electrically connected to the primary electrode and a secondaryconductor which is electrically connected to the secondary electrode.11. A sensor system according to claim 10 and further defined by saidmain lead signal cable being matched to have an impedance approximatelyequal to the combined impedances of the first and second leads.
 12. Asensor system according to claim 1 further comprising a main lead signalcable having a primary conductor which is electrically connected to theprimary electrode and a secondary conductor which is electricallyconnected to the secondary electrode, said main lead signal cable beingmatched to have an impedance approximately equal to the combinedimpedances of the first and second leads.
 13. A sensor system accordingto claim 12 wherein the first lead and second lead are approximately thesame length.
 14. A sensor system according to claim 1 and furthercomprising a vessel which defines the fluid channel therein;and whereinthe first and second sensors are mounted with said inside faces inopposing juxtaposition relative to one another across the fluid channel.15. A sensor system according to claim 14 wherein the sensing surfacesof said at least one primary electrode and the sensing surfaces of saidat least one secondary electrode are parallel.
 16. A sensor systemaccording to claim 14 wherein the sensing surfaces of said at least oneprimary electrode and the sensing surfaces of said at least onesecondary electrode are aligned in a plane.
 17. A sensor systemaccording to claim 1 wherein the sensing surfaces of said at least oneprimary electrode and the sensing surfaces of said at least onesecondary electrode are parallel and are aligned in the same plane. 18.A sensor system according to claim 17 and wherein the first sensor andsecond sensor are approximately axially aligned across the fluidchannel.
 19. A complementary pair of sensors used in a sensor system fortime domain reflectometry measurement of a fluid having constituentswith differing electrical permittivities, comprising:a first sensorhaving a first sensor housing and an inside face which is directedtoward the fluid during operation; at least one primary electrode madefrom electrically conductive material and having a sensing surface whichis along the inside face of the first sensor, the at least one primaryelectrode serving to conduct pulsed time domain reflectometry signalsalong the sensing surface of the at least one primary electrode; asecond sensor having a second sensor housing and an inside face which isdirected toward the fluid during operation; at least one secondaryelectrode made from electrically conductive material and having asensing surface which is along the inside face of the second sensor; afirst lead connected to the first sensor in electrical communicationtherewith, said first lead having a primary conductor and a secondaryconductor, said primary conductor of the first lead being electricallyconnected to the at least one primary electrode; a second lead connectedto the second sensor in electrical communication therewith, said secondlead having a primary conductor and a secondary conductor, saidsecondary conductor of the second lead being electrically connected tothe second electrode.
 20. A complementary pair of sensors according toclaim 19 and further comprising at least one dielectric layer coveringsaid sensing surfaces of the at least one primary electrode and at leastone secondary electrode, for electrically isolating the electrodes fromthe fluid mixture.
 21. A complementary pair of sensors according toclaim 19 wherein the at least one primary and secondary electrodes haveelongated sensing surfaces.
 22. A complementary pair of sensorsaccording to claim 19 and further comprising at least one transitionassembly providing electrical connection with at least one of saidprimary or secondary electrodes, and providing at least one seal to aidin preventing fluid leakage about said at least one of said primary orsecondary electrodes.
 23. A complementary pair of sensors according toclaim 19 and further comprising at least one transition assemblyproviding electrical connection with at least one of said primary orsecondary electrodes, the transition assembly including a transitioninsulator and an intermediate conductor which is in sealed relationshipreceived through the transition insulator to seal the sensor from thefluid mixture.
 24. A complementary pair of sensors according to claim 19wherein the first lead and second lead are approximately the same lengthand have approximately the same impedance.
 25. A complementary pair ofsensors according to claim 19 and further comprising at least onedielectric layer covering at least the sensing surface of the at leastone primary electrode, for electrically isolating the primary electrodefrom the fluid mixture and preventing dissipation of a time domainreflectometry signal propagated to the sensor system.
 26. Acomplementary pair of sensors according to claim 25 wherein the at leastone dielectric layer is an oxide layer formed upon the sensing surfaceof the at least one primary electrode.
 27. A complementary pair ofsensors according to claim 25 wherein the at least one dielectric layercovers portions of the at least one secondary electrode to electricallyisolate the at least one secondary electrode from the fluid mixture andto further prevent dissipation of a time domain reflectometry signal.28. A complementary pair of sensors according to claim 19 and furthercomprising a main lead signal cable having a primary conductor which iselectrically connected to the primary electrode and a secondaryconductor which is electrically connected to the secondary electrode.29. A complementary pair of sensors according to claim 28 and furtherdefined by said main lead signal cable being matched to have animpedance approximately equal to the combined impedances of the firstand second leads.
 30. A complementary pair of sensors according to claim19 further comprising a main lead signal cable having a primaryconductor which is electrically connected to the primary electrode and asecondary conductor which is electrically connected to the secondaryelectrode, said main lead signal cable being matched to have animpedance approximately equal to the combined impedances of the firstand second leads.
 31. A complementary pair of sensors according to claim30 wherein the first lead and second lead are approximately the samelength.
 32. A complementary pair of sensors according to claim 19 andfurther comprising a vessel which defines the fluid channel therein;andwherein the first and second sensors are mounted with said inside facesin opposing juxtaposition relative to one another across the fluidchannel.
 33. A complementary pair of sensors according to claim 32wherein the sensing surfaces of said at least one primary electrode andthe sensing surfaces of said at least one secondary electrode areparallel.
 34. A complementary pair of sensors according to claim 32wherein the sensing surfaces of said at least one primary electrode andthe sensing surfaces of said at least one secondary electrode arealigned in a plane.
 35. A complementary pair of sensors according toclaim 19 wherein the sensing surfaces of said at least one primaryelectrode and the sensing surfaces of said at least one secondaryelectrode are parallel and are aligned in the same plane.
 36. Acomplementary pair of sensors according to claim 35 and wherein thefirst sensor and second sensor are aligned across the fluid channel.